In current wireless communications systems, 5 MHz˜10 MHz radio bandwidths are typically used for up to 100 Mbps peak transmission rate. Much higher peak transmission rate is required for next generation wireless systems. For example, 1 Gbps peak transmission rate is required by ITU-R for IMT-Advanced systems such as the 4th generation (“4G”) mobile communications systems. The current transmission technologies, however, are very difficult to perform 100 bps/Hz transmission spectrum efficiency. In the foreseeable next few years, only up to 15 bps/Hz transmission spectrum efficiency can be anticipated. Therefore, much wider radio bandwidths (i.e., at least 40 MHz) will be necessary for next generation wireless communications systems to achieve 1 Gbps peak transmission rate.
Orthogonal Frequency Division Multiplexing (OFDM) is an efficient multiplexing scheme to perform high transmission rate over frequency selective channel without the disturbance from inter-carrier interference. There are two typical architectures to utilize much wider radio bandwidth for OFDM system. In a traditional OFDM system, a single radio frequency (RF) carrier is used to carry one wideband radio signal, and in a multi-carrier OFDM system, multiple RF carriers are used to carry multiple radio signals with narrower bandwidth. A multi-carrier OFDM system has various advantages as compared to a traditional OFDM system such as easier backward compatibility, better reuse on legacy single-carrier hardware design, more mobile station hardware flexibility, and lower Peak to Average Power Ratio (PAPR) for uplink transmission. Thus, multi-carrier OFDM systems have become the baseline system architecture in IEEE 802.16m (i.e. for WiMAX 2.0 system) and 3GPP Release 10 (i.e. for LTE-Advanced system) draft standards to fulfill system requirements.
In a multi-carrier OFDM system, however, it takes much longer time for a mobile station to perform scanning. First, a multi-carrier OFDM system typically supports two to four carriers in each cell, which will result in at least two to four times scanning time than a single-carrier OFDM system. Second, the number of cells in 4G systems will be much larger by the demands on more capacity to support higher throughput traffic and better received signal quality. This will lead to more microcells, picocells, and femtocells, in addition to macrocells, be deployed in 4G systems.
FIG. 1 (Prior Art) illustrates a traditional scanning process between mobile stations and multi-carrier base stations. In the example of FIG. 1, a single-carrier mobile station MS11 or a multi-carrier mobile station MS12 is scheduled with certain scanning intervals. For single-carrier base station BS13 supporting carrier #1, MS11 uses all the scanning intervals to scan carrier #1. For multi-carrier base station BS14 supporting two carriers #1 and #2, single-carrier MS11 uses half of the scanning intervals to scan carrier #1 and the other half of the scanning intervals to scan carrier #2. On the other hand, multi-carrier MS12 uses all the scanning intervals to scan both carriers #1 and #2. However, a multi-carrier MS often supports adjacent carriers, and a larger number of carriers will still result in less scanning intervals for each carrier. For example, for multi-carrier BS15 supporting four carriers #1-#4, single-carrier MS11 uses one-fourth of the scanning intervals to scan each carrier, while multi-carrier MS12 uses half of the scanning intervals to scan carriers #1 and #2, and the other half of the scanning intervals to scan carriers #3 and #4.
Therefore, the increase on the number of carriers will result in higher difficulty for an MS to complete scanning over each carrier for each BS. If the scanning opportunities remain the same, then the scanning result will be unreliable because less average duration is allocated for each BS over each carrier. This will degrade performance of certain procedures such as handover that rely on the scanning results. On the other hand, if the scanning opportunities are increased in response to the number of carriers, then the achievable user throughput will be degraded due to less transmission opportunities. This will result in difficulty for BS resource scheduling, especially when the number of users is increased. In addition, autonomous scanning cannot resolve this problem because the MS can only perform background scanning over the same carrier as the one used for data transmission. A solution is sought.